Method of tagging bulk materials



United States Patent Ofi ice 3,ll.3,i Patented Dec. 10, 1953 This invention relates to the identification or tagging of materials by non-radioactive techniques.

The use of radioactive tracers or tags is Well established. However, there are many limitations on the use of radioactive materials. These limitations include the health danger, the necessary monitoring equipment, and other problems raised by radiations from the radioactive material.

Accordingly, a principal object of the present invention is to identify or tag bulk materials Without the use of radioactive materials.

In accordance with the invention, the foregoing object may be achieved by the addition of traces of rare earth material to bulk material. Thus, for specific example, titanium metal may be tagged by the addition of less than 0.05 percent of yttrium or erbium. Preferably, both yttrium and erbium are added to the titanium, and the relative percentages of the tracer elements are different from the naturally occurring proportions of yttrium and erbium. Subsequently, the titanium and its tracer materials are reacquired and subjected to spectrographic analysis. In this manner the origin of the titanium material is ascertained beyond doubt. It is further noted that others of the rare earth materials may be used, and that they may be used with other bulk materials.

In accordance with a feature of the invention, traces of rare earth materials are added to bulk materials, the bulk material. is distributed, and the bulk material or a part of the bull; material is later recovered and subjected to spectrographic analysis.

In accordance with a featured article of manufacture, a tagged bulk material includes traces of at least two rare earths in proportions other than their naturally occurring proportions.

In accordance with additional features of the invention, the bulk material may be metallic and the rare earth materials may have a crystal structure and a melting point which are compatible with those of the bulk material to facilitate formation of a solid solution of the traces of rare earth and the bulk metal. In addition, the rare earth metals should have spectrographic lines which do not interfere with spectrographic matrix of the bulk material.

Other objects, features, and advantages of the invention will become apparent from a consideration of the following detailed description of certain illustrative embodiments and the underlying principles of my invention.

Before considering detailed examples embodying the principles of the invention, it is appropriate to consider the rare earths which are used as tracers and some of their properties. The rare earth materials include the elements of atomic numbers 57 through 71. Specifically, they include the elements lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium. One of the group, promethium, atomic number 61, does not occur naturally but is a fission product. Scandium and yttrium, atomic numbers 21 and 39, occur together With the rare earths in nature and are also group III-A elements. These last two elements are therefore generally included in the term rare earths and are so included in the present specification and claims. For the purposes of the present invention, the crystal structure and the melting point of the metals and the relative proportions of natural occurrence of the rare earths are particularly of interest. Accordingly, the best known values are set forth in the following table:

TABLE I Natural Occurrence; Approximate Parts pcr Million Crystal Melting Structure In the foregoing table, the crystal structure designations I-Icp, Cfc, Cbc and Rhom, stand for hexagonal, closepacked; cubic, faced-centered; cubic, body-centered; and rhombohedral; respectively.

NOW that the background for the invention has been set forth, a few specific examples will be presented.

Example I In four separate tests, titanium was tagged independently with yttrium and with erbium, with two different percentages for each tag. More specifically, 5 milligrams and 50 milligrams of yttrium were melted in an arc furnace in an inert atmosphere with 10 gram samples of titanium, to form the first two tagged samples; in two ad ditional examples, 5 milligrams and 50 milligrams of erbium were separately arc melted with 10 gram samples of titanium. The four samples of tagged titanium were subsequently subjected to spectrographic analysis. In the matrix of the titanium spectrum, the following spectrum lines can be used for identification:

TABLE II Yttrium, A. Erbium, A.

In the foregoing table, the symbol A. means angstrom unit, and this is a unit of length corresponding to one hundred-millionth of a centimeter, which is used principally in expressing the length of light waves.

In addition to the separate use of yttrium and erbium to tag titanium, it is also contemplated that they may be used together in a ratio which is different from that in which the elements occur naturally. Thus, for example, with reference to Table I, yttrium has a natural abundance of 28 parts per million and erbium has a natural occurrence of about four parts per million. Accordin ly, a ratio of one part of yttrium to one part of erbium would clearly distinguish the titanium bulk material from any possible contamination from natural sources.

Example II A chromium, molybdenum steel, United States Bureau of Standards Type 72e, was tagged with a similar combination of one part yttrium and one part holmium. In a first sample, the two additions together were added in a percentage of 0.03 percent by Weight the total material, and in a second sample the percentage was 0.3 percent. The yttrium and holmium spectrum lines which may be used for identification are as follows:

TABLE III Yttrium, A. Holmium, A.

Example III TABLE IV Neodymium, A. Samarium, A.

Example IV Ytterbium was used as a non-radioactive tagging addition to copper in an amount sufiicient to produce a concentration of 0.01 percent. A homogeneous sample of the ytterbium in copper was then analyzed spectrographically. A number of lines due to the presence of ytterbiurn were detected. The following three'representative lines can beused very adequately for identification of ytterbium in copper: 3289.4 A., 3694.2 A., 398 8.0 A. These uniquely identify ytterbium with no interference from the copper matrix.

In Table l the relative natural occurrence of the various rare earths in parts per million in the earths surface is set forth. It is noted, however, that the relative percentages in the principal known concentrated deposits of the rare earth metals differ somewhat from the figures of Table I. These deposits include monazite, bastnasite and xenotirne. Rare earths are also obtained as a by-product in the process of obtaining uranium, colurnbiurn and tantalum from euxenite or other minerals. The rare earth elements always occur naturally as mixtures rather than as individual elements, and the relative proportions of the elements which are present are normally in rough agreement with the values given in Table I. When two or more rare earths are used as a tag, therefore, it is desirable to use a ratio of the rare earths which is significantly higher or lower than the ratio as derived from Table I.

In addition to the relative abundance, Table I shows the crystal structure and the melting point of each of the rare earth metals. To facilitate the tagging of individual metals, it is desirable that the melting points of the rare earth materials be compatible with that of the bulk metal or other material which is being employed. Furthermore, to minimize interference with the mechanical or other properties of the bulk material, the rare earth material may be chosen to have the same crystal structure as the bulk material which is being tagged. Thus, with reference to one of the examples set forth above, yttrium, erbium and titanium all have a close packed hexagonal crystal structure. With the very small percentages such as about .05 percent which are employed, however, the presence of the tagging material will generally produce little adverse eilect, even if it has a different crystal structure.

With regard to the percentages of rare earth materials which should be used, it is generally desirable to use traces of about 1.0 to .001 percent by weight of the rare earth materials, with the range of about 0.5 to .01 percent being preferred. In some cases as little as .0001 percent may be used where the trace materials have prominent spectrum lines in the matrix being examined. In addition, in cases where the bulk material may subsequently be physically dispersed in some manner, percentages, of the rare earths may initially be as high as 5 to 10 percent. This latter case might involve the preparation of a master alloy to be used in the preparation of other alloys, for example. I i

It is recognized that rare earth materials have been used as additives heretofore, to improve the properties of metals and for other reasons. In the present case, however, the rare earths are to be used as tags to identify bulk materials. Thus, for example, a producer of steel, aluminum or of alloy metals may Wish to tag his metals. Then, at some later date, upon the occurrence of a failure of a fabricated metal part, the producer may readily determine whether or not the metal was originally produced by him. This is verified by spectrographic analysis of a small sample of the metal part, to check for the original rare earth tags.

It is further noted that rare earth materials may be added in the pure form to bulk materials. Under these circumstances, it may be desirable to prevent oxidation of the rare earths by heating in a vacuum or in an inert atmosphere, or otherwise. Alternatively, the rare earth tags may be added as compounds such as the oxides. This might be appropriate, for example, in the tagging of glass, which is largely silicon oxide.

It is to be understood that the above described arrangements are illustrative of the application of the principles of the invention. Numerous other arrangements may be devised by those skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

l. The method of identifying bulk material which comprises selecting at least two rare earth materials each having spectrographic lines which do not interfere with the spectrographic matrix of the bulk material and each having a melting point compatible with the melting point of said bulk material, adding a trace amount of only said selected rare earth materials to the bulk material, the relative proportions of the selected rare earth materials added being significantly different from the proportions in which they occur in naturally occurring deposits and melting the mixture, solidifying the molten material thereby forming a solid solution, distributing the solidified material, recovering a portion of the distributed material and subjecting the recovered material to spectrographic analysis to determine the particular rare earth materials and the amount of each present therein.

2. A process as recited in claim 1, wherein the bulk material has a crystalline structure and two rare earth metals are added, each of which has a crystal structure of the same general type as that of the bulk material.

3. A process as recited in claim 1, wherein all of the materials are metals and two rare earth metals in the ratio of 1:1 are added to the bulk metal.

4. A process as recited in claim 3, wherein the bulk metal is titanium and the rare earth metals are yttrium and erbium in the ratio of 1:1.

5. A process as recited in claim 3, wherein the bulk material is steel and the rare earth metals are yttrium and holmium in the ratio of 1:1.

6 6. A process as recited in claim 3, wherein the bulk metal is an aluminum magnesium alloy and the rare earth metals are neodymium and samarium in the ratio of 1:1.

References Cited in the file of this patent UNITED STATES PATENTS Dickerson Mar. 3, 1959 Lucien et al Apr. 14, 1959 OTHER REFERENCES 

1. THE METHOD OF IDENTIFYING BULK MATERIAL WHICH COMPRISES SELECTING AT LEAST TWO RARE EARTH MATERIALS EACH HAVING SPECTROGRAPHIC LINES WHICH DO NOT INTERFERE WITH THE SPECTROGRAPHIC MATRIX OF THE BUILK MATERIAL AND EACH HAVING A MELTING POINT COMPATIBLE WITH THE MELTING POINT OF SAID BULK MATERIAL, ADDING A TRACE AMOUNT OF ONLY SAID SELECTED RARE EARTH MATERIALS TO THE BULK MATERIAL, THE RELATIVE PROPORTIONS OF THE SELECTED RARE EARTH MATERIALS ADDED BEING SIGNIFICANTLY DIFFERENT FROM THE PROPORTIONS IN WHICH THEY OCCUR IN NATURALLY OCCURRING DEPOSITS AND MELTING THE MIXTURE, SOLIDIFYING THE MOLTEN MATERIAL THEREBY FORMING A SOLID SOLUTION, DISTRIBUTING THE SOLIDIFIED MATERIAL, RECOVERING A PORTION OF THE DISTRIBUTED MATERIAL AND SUBJECTING THE RECOVERED MATERIAL TO SPECTROGRAPHIC ANALYSIS TO DETERMINE THE PARTICULAR RARE EARTH MATERIALS AND THE AMOUNT OF EACH PRESENT THEREIN. 